1. Technical Field
This invention relates to a method for optical transmission over a fiber optic network and to an optical communications network as set forth in the preambles of the respective independent claims.
2. Discussion of Related Art
When optical pulses are transmitted over fiber optic links, chromatic dispersion occurs. If, for example, a single bit is respresented by an optical pulse with a duration of 25 ps, this Gaussian pulse broadens after a 100-km fiber length to approximately 240 ps and, in addition, has a continuously varying frequency. Chromatic dispersion corrupts the data transmitted and limits the optical link lengths. To be able to cover long transmission distances and receive usable signals in the receiver despite the chromatic dispersion, use is made of dispersion-compensating optical elements. From M. Kakui et al, "2.4-Gbit/s, 306-km repeaterless transmission . . . ", OFC'95 Technical Digest, page 148, for example, it is known to use dispersion-compensating fibers to compensate for the chromatic dispersion of the transmission link.
The dispersion-compensating optical elements compress the broadened pulses of the data stream back into the original bit pulses of 25 ps full width at half maximum and make the signal detectable for the subsequent photodiode. The dispersion-compensating elements may be conceived as optical correlation filters which correlate the distorted data bits out of the received data stream.
From N. Takachio et al, "Optical PSK Synchronous Heterodyne Detection . . . ", IEEE Photonics Technology Letters, Vol. 4, No. 3, 1992, page 278, a method is known for detecting optical signals which were degraded by chromatic dispersion on the transmission link. In the receiver, a heterodyne detection method is used. The received optical signal is combined with local oscillator light by a fiber coupler and detected by a balanced detector. A problem with such heterodyne detection is that the optical fields of the local oscillator and the input signal beat together optimally only if they are spatially coherent and have the same polarization. Even minor disturbances of the oscillator frequency and/or data clock deviations, particularly during transmissions at high data rates, result in spurious modulations being superimposed on the signal, which make any further evaluation impossible. The big advantage of heterodyne reception is, however, that both the amplitude and phase of the incoming optical signal are also contained in the photodiode signal. Therefore, distortion due to chromatic dispersion can be eliminated with a linear electrical filter. Thus, even if a dispersive fiber is used for the fiber optical link, the transmission distance can be substantially increased.